Cavern Depth Research Articles

Study on salt crystals fouling on the tubing surface for energy storage salt cavern during debrining

Salt crystals fouling may lead to the blockage of debrining inner tubing(DIT) for underground gas storage(UGS) salt cavern. In this paper, the components of salt crystals are analyzed, and a model for calculating the salt crystals fouling is established, which considers the coupling effects of flow-thermal-chemical. This model is verified by comparing monitoring data of an actual salt cavern debrining. The influences of salt cavern depth, DIT size and heat conductivity on salt crystals fouling rate are analyzed. When the salt cavern depth is increased from 1000 to 2000 m, the salt crystals fouling rate increases from 3.61 to 32.17 mm/day. The influences of the DIT size on the salt crystals fouling rate are minor. When the heat conductivity of tubing decreases from 4 to 0.4 W/(m·℃), the salt crystals fouling rate reduces by 30.37 %. This study could help formulate a debrining scheme to prevent the DIT clogging.

Investigation on friction pressure of oil storage and brine discharge in underground salt caverns

The design and control of oil injection and brine removal constitute a crucial task in the construction of salt cavern oil storage (SCOS). In this article, the innovation is that the SCOS pressure governing equations were established, and the impacts of salt cavern depth, crude oil viscosity, as well as injection rate on pipeline friction pressure and wellhead injection pressure were calculated and assessed. Besides, the selection principle of varying viscosity crude oil in different depths of salt caverns, as well as the implementation of an optimal oil injection rate, were proposed. The findings indicated that within the depth range of 500 ∼ 2,000 m, there was a nearly linear correlation between the increase in salt cavern depths and the corresponding rise in injection pressures. The friction pressure generated by the injection pipe constituted the predominant proportion among them, and the range of oil injection pressure was 3.4 ∼ 23.0 MPa. Additionally, the injection pressure and friction pressure were primarily influenced by the viscosity of crude oil, with cavern depth and injection displacement following suit. Shallow salt caverns are optimal for storing high-viscosity crude oil, whereas deep salt caverns are better suited for low-viscosity crude oil storage.

An analytical solution for elastoplastic responses of a lined rock cavern for compressed air energy storage considering excavation and high internal pressure

An analytical solution for elastoplastic responses of an underground lined rock cavern for compressed air energy storage under initial hydrostatic pressure and high internal air pressure is proposed. In the analytical solution, the stress paths during cavern excavation and operational stages are considered. A numerical simulation is performed by using the finite element software Abaqus to verify the analytical solution. The concept of “high-pressure plastic zone” is proposed and the evolution of “high-pressure plastic zone” is analyzed. Furthermore, taking the occurrence of “high-pressure plastic zone” and failure of the sealing layer as the critical states, the calculation methods for critical internal pressure pcr1 and ultimate internal pressure pu are proposed. Sensitive analyses are conducted to study the key parameters affecting the “high-pressure plastic zone” and the critical internal pressure. The research indicates that the proposed analytical solution can accurately predict the elastoplastic responses of an underground lined rock cavern. The radius of “high-pressure plastic zone” are mainly related to the surrounding rock quality, magnitude of air internal pressure and buried depth of cavern. The critical internal pressure pcr1 and ultimate internal pressure pu are mainly related to the surrounding rock quality, cavern size, buried depth and thickness of concrete layer.

Characteristics and Classification of Choroidal Caverns in Patients with Various Retinal and Chorioretinal Diseases

To investigate the features of choroidal caverns in diverse retinal diseases with swept-source optical coherence tomography (SS-OCT). Subjects with normal eyes, retinitis pigmentosa (RP), wet age-related macular degeneration (wAMD), acute central serous chorioretinopathy (CSC), or chronic CSC were enrolled. The characteristics of choroidal caverns were evaluated with SS-OCT. The prevalence of choroidal caverns in retinal diseases and the correlations between the number, width and depth of choroidal caverns with the thickness of choroid were analyzed. Among 315 eyes of 220 subjects, choroidal caverns were found in 110 eyes (34.9%). Choroidal caverns were divided into two categories based on their location and size. Type I was small and usually lobulated, presented in the choroidal capillary and Sattler's layers. Type II was larger, usually isolated, and presented in the Sattler's and Haller's layers. The prevalence of type I in subjects with normal eyes, RP, wAMD, acute CSC, or chronic CSC was 17.4%, 19.6%, 1.6%, 32.8%, and 85.2%, respectively, while that of type II was 0%, 0%, 21.3%, 13.8%, and 53.7%, respectively. The number, width, and thickness of type II choroidal caverns correlated positively with macular choroidal thickness. Choroidal caverns could be divided into two categories. Type II choroidal caverns appeared associated with the pachychoroid spectrum and RPE atrophic diseases.

Plasma-Beam Processing of Tools Made of SiAlON Dielectric Ceramics to Increase Wear Resistance When Cutting Nickel–Chromium Alloys

This research aimed at an increase in wear resistance of round cutting plates manufactured with SiAlON dielectric ceramics through deposition of wear-resistant coatings. To increase effectiveness of the coatings, their adhesion was improved by the removal of defective surface layers from the cutting plates before the deposition. As the depth of caverns and grooves appearing on the cutting plates due to manufacturing by diamond grinding reached 5 µm, a concentrated beam of fast argon atoms was used for the removal of defective layers with a thickness exceeding the depth of caverns and grooves. At the equal angles of incidence to the front and back surfaces of the cutting wedge amounting to 45 degrees, two-hour-long etching of rotating cutting plates provided removal of defective layers with thickness of ~10 µm from the surfaces. After the removal, the cutting edge radius of the plates diminished from 20 to 10 µm, which indicates the cutting plates’ sharpening. Wear-resistant TiAlN coatings deposited after the etching significantly improve the processing stability and increase wear resistance of the cutting plates by not less than 1.7 times.

Surface Porosity of Natural Diamond Crystals after the Catalytic Hydrogenation

The study of diamond surfaces is traditionally undertaken in geology and materials science. As a sample material, two natural diamond crystals of type Ia were selected, and their luminescence and nitrogen state was characterized. In order to etch the surface catalytic hydrogenation was performed using Fe particles as an etchant. Micromorphology of the surface was investigated by scanning electron and laser confocal microscopy. It was demonstrated that etching occurred perpendicular to the crystal surface, with no signs of tangential etching. The average depth of caverns did not exceed 20–25 μm with a maximal depth of 40 μm. It is concluded that catalytic hydrogenation of natural type Ia diamonds is effective to produce a porous surface that can be used in composites or as a substrate material. Additionally, the comparison of results with porous microsculptures observed on natural impact diamond crystals from the Popigai astrobleme revealed a strong resemblance.

Geometric Model for Physical Simulation System of Coal and Gas Outburst

The physical simulation system is an important means to study the mechanism of coal and gas outburst. The small- and medium-sized coal and gas outbursts account for the largest proportion in China. So, the paper focuses on small- and medium-sized coal and gas outbursts based on key physical rules and their statistical characteristics. According to the geometric parameters statistical data of more than 100 caverns, the proportion of outburst caverns whose depth is less than 5 m is 80%, and whose depth is more than 6 m is 13%. The ratio of depth and cavern outlet’s diameter is 2.58–7.31. There are statistical relationship of 2 times between coal’s and cavern’s volume, and between gas emission volume per ton and gas content in seam. Then, the cavern of coal and gas outburst is simplified as an ellipsoid, and the shape of cavern outlet is simplified as a circle. The cavern’s and the prototype’s geometrical parameters of small and medium coal and gas outbursts were deduced. The depth of cavern is 5 m, width is 4 m, and diameter of cavern outlet is 0.68–1.94 m. The depth of coal and outburst prototype is 12.3 m, width is 6.5 m, and diameter of cavern outlet is 0.68 m. In order to guarantee the movement similarity, according to the relationships among statistical data of outburst coal particle’s size, cavern outlet’s size, and height of tunnel, the minimum cavern outlet’s size was calculated. Select 10 times the coal particle’s size (5 mm) as the smallest size. Then, maximum geometric similarity ratio of coal and gas outburst was deduced, and it is 13.6. The minimum geometrical parameters of coal and outburst similar model were obtained. Its depth is 0.9 m, width is 0.48 m, and diameter of cavern outlet is 50 mm. According to the results, the physical simulation system was developed. The results provide support to carry out physical simulation experiment of coal and gas outburst.

Stability Analysis of Salt Cavern Gas Storage Using 2D Thermo-Hydro-Mechanical Finite-Element Software

Ensuring the stability and integrity of underground gas storage salt caverns is a very complicated subject due to the non-linear and time-dependent behavior of rock salts under complicated thermal and mechanical loading conditions. For this reason, pressure and temperature fluctuations in the caverns and their surrounding strata must be integrated into the analysis and the numerical tools that are used for this purpose. LOCAS, a 2D axisymmetric finite-element code, dedicated to the stability analysis of underground salt spaces, was applied to assess the effects of various operating and geometrical parameters on the cavern behavior. In this paper, we aimed to give an overall assessment of the behavior of the salt caverns used for natural gas storage. In this work, some specific loading scenarios were considered first, followed by thorough parametric and sensitivity analyses to reveal the impacts of the geometrical parameters and operational parameters involved on the behavior of salt caverns using the modern stability criteria. The findings showed that the onset of dilation was more likely to happen within the first cavern life cycle when pressure dropped to the minimum level. As for the potential of tension occurrence in the surrounding rock, this is more likely to happen by increasing the number of operation cycles, especially in the upper one-third of the cavern wall. Finally, it was seen that the cavern depth and minimum cavern internal pressure had even more important influences than the others on the salt cavern behavior.

Analysis of the Prospects of Ultrajet Diagnostics for Estimating the Wear Resistance of Bimetallic Tool

Ultrajet diagnostics is used to analyze bimetallic knives for rotary crushers, and a polymer powder is introduced in a jet to imitate the operating conditions of the equipment. The correlation between the cavern depth formed during ultrajet action on a knife and the geometric parameters of the wear of its cutting edge is shown to be high.

Cases, causes and classifications of craters above salt caverns

Based on the description of a dozen of cases, this paper suggests a categorization of craters formed above brine-production caverns for “piston” and “hourglass” types. The deliberate creation of three craters in Lorraine (France) above the Keuper salt formation is described first. After the cavern roof reaches the top of the salt formation, stoping takes place. A rigid cylinder of rock (a piston) drops abruptly in the cavern, experiencing no deformation and creating vertical crater edges. In the same mining district, a room-and-pillar panel collapsed abruptly in 1873. Multiple pieces of evidence have proven that, here again, no deformation occurred in the cylinder that dropped into the mine. These examples prove that cavern drop is more abrupt when the cavern is filled partly with air, as less brine must be evacuated from the cavern during collapse. The Haoud Berkaoui (Algeria) and Bereznikovsky (Russia) craters also belong to the piston type; less information is available. The main features of the piston mechanism can be captured by a simple model: failure takes place when vertical shear forces along the cylinder edge, plus cavern internal pressure, are not able to balance cylinder weight. This model suggests that the contour of the crater must be a circle and that collapse is easier when the ratio between cavern radius and cavern depth is larger. This may explain why no example of a collapse above a hydrocarbon storage cavern is known: in most cases, this ratio is very small. Three sinkholes formed above salt caverns leached out from the Hutchinson salt formation in Kansas (USA) epitomize the hourglass type. Here, again, stoping occurs until the cavern roof reaches loose sediments at shallow depth. A sinkhole grows when sediments flow to a central hole to fill the cavern underneath, generating an upward flow of brine to the sinkhole. Such a phenomenon also can occur in a salt dome. At Bayou Choctaw (Louisiana, USA) a brine cavern rose through the caprock, allowing loose shallow sediments to flow to the cavern. At Bayou Corne (near Napoleonville in the same state), a cavern was within a short distance from the flank of a dome; a 1500-m-deep breach was created, and loose sediments in the dome sheath, accumulated during geological times, filled the cavern—a process that lasted more than one year and created a sinkhole at ground level. In both cases (piston and hourglass) lakes form in the crater and gravity-driven waves are observed. Sinkhole creation can be prevented when the distance between the cavern roof and the salt top (or dome flanks) is large enough and when the ratio between cavern radius and cavern depth is small enough.

Geometrical versus rheological transient creep closure in a salt cavern

An in-situ test performed in a brine-filled cavern proves that, when brine pressure decreases rapidly, the creep closure rate increases drastically. Conversely, a rapid pressure increase leads to “reverse” creep closure: cavern volume increases, even when, at cavern depth, fluid pressure is lower than geostatic pressure. It is tempting to explain these two phenomena by transient salt creep, a characteristic feature of salt rheological behavior commonly observed during laboratory creep tests. In fact, computations performed on an idealized cylindrical cavern excavated from a Norton–Hoff rock mass (a constitutive law that includes no transient component) prove that these two phenomena are, at least partly, of a structural nature: their origin is in the slow redistribution of stresses following any pressure change.

Analysis of the behavior of large underground oil storage caverns in salt rock

Summary The storage of petroleum products above ground surface has many constraints and limitations. A viable alternative is to excavate large underground spaces in rock to provide a safer way for oil storage. Soft rock formations such as salt domes provide suitable conditions from environmental and operational aspects. The potential for high volume storage and low permeability are among advantages of oil storage in caverns excavated in salt rocks. The complicated shape of oil storage caverns, complex behavior of salt rock, and boundary conditions associated with large underground openings are major challenges in the design of salt caverns excavated for oil storage purposes. In this study, the deformation mechanism and stability of salt caverns were investigated. A comprehensive 3D numerical study was carried out to investigate the effects of cavern size and depth, salt rock deformation modulus, and ground in-situ stress regime on the behavior of large salt caverns. The stress field and deformation mechanisms were studied numerically aiming at shedding lights into the design aspects of salt caverns for oil storage. The analysis results show that the cavern safety factor is reduced as a function of cavern depth and storage volume. Also, with decrease in k (ratio of horizontal to vertical in-situ stress), the stability of salt caverns will increase; however, with increase in salt rock young modulus, the sensitivity of cavern stability to k ratio is reduced. Copyright © 2016 John Wiley & Sons, Ltd.

Time-dependent subsidence prediction model and influence factor analysis for underground gas storages in bedded salt formations

Taking into account the impurity and creep of rock salt, a new model is proposed to predict the time-dependent subsidence of the surface above underground gas storage caverns in bedded salt formations. In this model, the shape of the storage cavern is assumed to be a sphere. Formulae are developed for the volume and volume reduction of the cavern over time during both the construction and the operating periods. A probability integral method is adopted to determine the shape of the subsidence regions, and corresponding formulae are derived for the time-dependent surface subsidence, radial tilt and radial curvature. FLAC3D was used to verify the proposed prediction model. Using the proposed model, parametric analyses are carried out to study the influences of draw angle, volume adjusting coefficient, brine withdrawal rate, internal pressure, cavern depth, material properties of the rock salt, cavern diameter, and operating time on the surface subsidence. The results show that: the subsidence increases with an increase of the draw angle, volume adjusting coefficient, brine withdrawal rate, cavern diameter, material properties of the rock salt, and time, but decreases with an increase of internal pressure. Subsidence increases first and then decreases with increasing cavern depth. Draw angle, volume adjusting coefficient, cavern depth, cavern diameter, material properties of the rock salt, time and internal pressure have a large influence on the subsidence, but there is little influence from the brine withdrawal rate and salt impurity.

Analysis of the Possibility of Carbon Dioxide Storage in China’s Underground Salt Caverns

The underground salt caverns created by solution mining used for storage has great advantages over other storage methods. However, not every underground salt cavern created in China is suitable for CO2 storage owing to different reasons like water resources and the depth of salt caverns. The author searched for the geographic information of salt layers in China first. Secondly , through general analysis like analyzing salt caverns’ location and plate, some salt layers good for CO2 storage are listed. Comparing to a case studying from Australia's Otway Basin, which applies underground gas storage experience to geological carbon dioxide storage, the author analyzed the possibility of CO2 storage in Jingtan,China from different aspects mainly about the leakproofness and salt caverns stability. Some suggestions are given concerning the construction of CO2 storage in underground salt caverns at last.

Analysis of Pillar Stability in Thermal Energy Storage Caverns Using Numerical Modeling

Underground Thermal Energy Storage (UTES) systems store thermal energy in aquifers, boreholes, or caverns with the purpose of avoiding the sporadic characteristics of renewable-energy resources or recovering industrial waste heat. Cavern TES (CTES) system may be more expensive in construction than the other two storage methods, but its advantages include high injection and extraction powers and the flexibility in selecting a heat storage medium. The first large-scale cavern for CTES system, which is toroidal shaped, was constructed for storing hot water of 40–90 C in Lyckebo, Sweden and 5,500 MWh of heat was stored between seasons (Nordell et al. 2007). Storage cavern in CTES system should be designed to ensure structural stability of storage space and provide good thermal storage performance as well. However, these two objectives conflict with each other, because increasing height-to-width ratio of the cavern improves the storage performance and simultaneously increase the structural instability due to its slender shape (Park et al. 2013). It can be thus suggested to divide a single large cavern into multiple smaller caverns with high aspect ratios (Park et al. 2014), and cavern spacing affecting the temperature and stress distribution of the pillar between caverns becomes one of the main factors taken into account when designing the multiple caverns. Generally, closely spaced caverns are preferred for efficient placement of surface and underground facilities and favorable to reduce the heat loss due to the temperature difference between surrounding rock mass and storage medium. This technical note seeks to determine a width of the pillar that is structurally stable between two rock caverns for thermal energy storage, based on numerical simulations using a thermal–mechanical coupled model. The influence of pillar width on the stability is compared with that of cavern depth and in situ stress condition, and the stress distributions of pillars with different widths are analyzed.

열-역학적 연계해석 모델을 이용한 다중 열저장공동 안정성 분석

암반공동을 이용한 열에너지 저장은 대용량 저장이 가능하며 열저장매체를 선택할 수 있는 장점이 있다. 본 연구에서는 사일로 형태의 열저장공동이 지반 내 두 개 이상 배치될 때 공동 사이에 형성되는 암반 필라의 안정성에 대해 3차원 유한차분해석 프로그램인 <TEX>$FLAC^{3D}$</TEX>를 이용하여 분석하였으며, 저장된 열에너지로 인해 암반에 발생하는 열응력을 반영할 수 있도록 열-역학적 연계모델을 사용하였다. 해석 결과, 열에너지 장기 저장으로 인해 암반 필라에 작용하는 최대주응력이 상당량 증가하였으며, 필라 폭이 좁아질수록 근접한 열원 때문에 열응력 증가량도 커짐을 확인하였다. 필라 안정성에 영향을 미치는 주요인자로서 저장공동 간격, 측압계수, 심도를 선정하고 민감도 분석을 실시한 결과, 측압계수, 저장공동 간격, 심도 순서로 영향력이 크게 평가되었다. 저장공동 간격의 경우 동일한 크기의 공동 건설 시 필라 폭을 최소 저장공동 직경 이상 확보해야 할 것으로 판단되었다. 큰 규모의 저장공동 주변에 소규모 수직갱이 설치될 때는 최소한 저장공동 직경의 0.5배 이상 이격함으로써 크기 차이로 인해 수직갱에 응력이 집중되는 현상을 해소할 수 있었다. 또한 최대수평주응력 작용방향과 공동 중심을 잇는 축이 평행하도록 배치하여 저장공동에 의한 방패효과가 발휘될 수 있게 함으로써 현지응력이 공동 사이 암반 필라에 미치는 영향을 최소화할 수 있었다. Cavern Thermal Energy Storage system stores thermal energy in caverns to recover industrial waste heat or avoid the sporadic characteristics of renewable-energy resources, and its advantages include high injection-and-extraction powers and the flexibility in selecting a storage medium. In the present study, the structural stability of rock mass pillar between these silo-type storage caverns was assessed using a coupled thermal-mechanical model in <TEX>$FLAC^{3D}$</TEX>. The results of numerical simulations showed that thermal stresses due to long-term storage depended on pillar width and had significant effect on the pillar stability. A sensitivity analysis of main factors indicated that the influence on the pillar stability increased in the order cavern depth < pillar width < in situ condition. It was suggested that two identical caverns should be separated by at least one diameter of the cavern and small-diameter shaft neighboring the cavern should be separated by more than half of the cavern diameter. Meanwhile, when the line of centers of two caverns was parallel to the direction of maximum horizontal principal stress, the shielding effect of the caverns could minimize an adverse effect caused by a large horizontal stress.

An optimization model for managing compressors in salt cavern gas storage

In light of the upcoming gas market liberalization, the storage in underground gas stor‐ age should be treated as a commercial activity, providing profit to investors. Gas fuel con‐ sumed for the operation of underground gas storage facility, mainly by work of compressors injecting gas into storage, will have to be carefully accounted and bought at market prices. In a competitive gas market, technical and economic optimization of the underground gas storage operation takes on particular significance. Liberalized market requires that the under‐ ground gas storage operator manages the operation in the most efficient way, affecting the profitability of service storage provision. Optimization of operation of underground gas storage in salt cavern (CUGS) is an im‐ portant operational problem in CUGS management. Despite many works on this subject [6, 16, 17, 20], it is not totally resolved. The reduction of gas storage capacity in a bed of rock salt, called convergence is inevitable, but its size also depends on the cavern depth and the operating storage [5, 19]. Operation practice suggests that the first step is to inject a large cavern convergence, then the lower and finally the shallowest cavern lying. In the withdraw process, the situation is reversed. However, the optimization that takes into account only the speed of storage caverns convergence is not ideal. You may find that maintaining a regime of operation that minimizes the effect of reducing the volume of the storage spaces can increase the cost of the gas installation (surface infrastructure). It is because Injection of the gas to the deepest caverns requires more power compressors. This occurs in CUGS Mogilno where the system can transport up to 400,000 standard cubic meter per hour [SCm/hr] of natural gas which about 2% is used by the compressor sta‐ tions to inject gas into storage caverns. It is estimated [13] that the global optimization of the

Investigating the effects of lateral stress to vertical stress ratios and caverns shape on the cavern stability and sidewall displacements

Effects of lateral stress to vertical stress ratio on behavior of a cavern in various geomechanical and geometrical conditions were studied. Results indicated that the range of one to two lateral stress to vertical stress ratios was the best condition for cavern stability. The ranges causing tension and compressive failure were specified as well. Two-dimensional stability analyses were carried out by using Phase2. Key point location on the cavern side wall was investigated and determined using an equation based on a large number of numerical analyses. Subsequently, in order to predict the elasto-plastic displacement and elastic displacement on a side wall key point, two equations were fitted based on various cavern cross sections considering four basic factors, i.e., rock deformation modulus, overburden depth of caverns, heights of the caverns, and the lateral stress to vertical stress ratio. The proposed equations were utilized to predict displacement at the key points of 10 projects subsequent to which the computation results were compared to in-site measuring results and back analysis results. Finally, using key point displacement as a stability factor, the effects of three different shapes of caverns including mushroom, horse shoe, and elliptical were investigated on cavern stability. The most optimum shape was elliptical in a vast range of lateral stress to vertical stress ratios; mushroom and horse shoe shapes were preferred in uniaxial stress fields concerning the rock quality.

Parametric sensitivity analysis of ground uplift above pressurized underground rock caverns

In this paper, we investigated the effect of design parameters of pressurized underground rock caverns on the safety against ground uplift using a simple mathematical solution. The solution was derived from a limit equilibrium analysis where a straight failure plane to ground surface is assumed above the cavern and shear resistant force on the failure plane induced by cohesion and friction angle of overburden rock mass is considered. Parametric sensitivity analysis considering storage pressure, cavern size, overburden depth, in-situ stress state, and rock physical properties was carried out and an exemplary design chart for a pressurized underground cavern was presented, providing with the preliminary information on the acceptable ranges of storage pressure, cavern depth and radius for the cavern safety against ground uplift.

Stability analysis of the pillars between bedded salt cavern gas storages by cusp catastrophe model

The failure of pillars between bedded salt cavern gas storages can be seen as processes that the deformations of pillars convert from continuous gradual change system to catastrophe state, which are typical nonlinear catastrophe problems. In the paper, the cusp catastrophe model is proposed to obtain the stability factors of pillars. It can overcome the shortages of traditional strength reduction finite element method (SR FEM) and greatly improve the accuracy of stability factors obtained by numerical simulations. The influences of cavern depth, gas pressure, pillar width, and time on the stability factors are studied. Y-1 and Y-2 salt cavern gas storages, located at Jiangsu province of China, were simulated as examples. The stability factors of pillars between Y-1 and Y-2 were evaluated, and the running parameters were recommended to ensure the pillars stability. The results showed that the cusp catastrophe model has high practicability and can precisely predict the stability factors. The stability factors are equidirectional with the increase of gas pressure and pillar width, but reverse to the increase of cavern depth and time. The stability factors of pillars between Y-1 and Y-2 are small for narrow widths, which are influenced greatly by gas pressure, time, pressure difference, and gas production rate. In order to ensure the safety of pillars, the lowest gas pressure, safe running time, max. pressure difference and max. gas production rate of Y-1 and Y-2 were recommended as 7 MPa, 5 years, 3 MPa, and 0.50 MPa/d, respectively.